1. Field of the Invention
This invention relates to methods and systems for locally sealing a vacuum microcavity, methods and systems for monitoring and controlling pressure and methods and systems for trimming resonant frequency of a microstructure therein.
2. Background Art
Packaging is one of the most challenging and costly processes in the IC and sensor industry. Despite the enormous efforts and technological advances in this area during recent years, IC packaging remains a bottleneck for IC commercialization. With moving components, the already challenging packaging task is complicated further for MEMS devices. Thus far, packaging methods reported in the literature are application specific, and there is a lack of standardization in wafer-level MEMS device packaging. Furthermore, high, unpredictable, residual pressures inside the cavity and vacuum degradation over time are major obstacles in packaging high-performance MEMS devices such as high-quality factor resonators and absolute pressure sensors. For example, a prior art absolute pressure sensor is capable of resolving pressure changes of 25 mTorr and can be packaged at wafer level; however, it has much lower absolute sensitivity due to the variable pressures in the reference cavity.
Reactive Sealing
One of the earliest and best reactive sealing techniques was described by Guckel and Burns in their article “Planar Processed Polysilicon Sealed Cavities for Pressure Transducer Arrays”, IEEE International Electron Devices Meeting, TECHNICAL DIGEST, IEDM '84, pp. 223-225, San Francisco, 1984. A cavity with a thin gap, which is defined by a sacrificial layer, was sealed with thermal oxidation. Sealed cavities (or microshells) can also be formed by deposited films. LPCVD films are typically used because of their excellent step coverage, as described by L. Lin, K. McNair, R. T. Howe, and A. P. Pisano in “Vacuum Encapsulated Lateral Microresonators,” DIGEST OF TECHNICAL PAPERS, 7th International Conference on Solid State Sensors and Actuators, pp. 270-273. Yokohama, June 1993. This sealing method is also referred to as shadow plugging, because the deposited thin film does not coat the interior of the cavity.
Localized Heating and Bonding
Localized heating and bonding have been utilized to package MEMS devices by Cheng et al. as described in the following documents:                Y. T. Cheng, L. Lin and K. Najafi, “Localized Bonding with PSG or Indium Solder as Intermediate Layer”, PROC. OF IEEE INTERNATIONAL CONFERENCE ON MICRO ELECTRO MECHANICAL SYSTEMS, pp. 285-289, Orlando, 1999;        Yu-Ting Cheng, “Localized Heating and Bonding Technique for MEMS Packaging”, Ph.D. Dissertation, University of Michigan, 2000;        Y. T. Cheng, L. Lin and K. Najafi, “Fabrication and Hermeticity Testing of a Glass-Silicon Packaging Formed Using Localized Aluminum/Silicon-to-Glass Bonding”, INTERNATIONAL MEMS CONFERENCE, pp. 757-762, 2000; and        L. Lin, “MEMS Post-Packaging by Localized Heating and Bonding”, IEEE TRANSACTIONS ON ADVANCED PACKAGING, Vol. 23, No. 4, pp. 608-616, Nov. 2000.        
A microheater defines the perimeter of the cavity, and is used to raise the temperature locally in order to achieve one of the following: (1) reflow the heater material itself, such as Al, to seal the cavity; (2) reflow an intermediate layer, such as PSG or solder, to seal the cavity; or (3) raise the temperature locally to induce a chemical reaction between adjacent materials to seal the cavity (localized fusion bonding and eutectic bonding).
Cavity Pressure Control Using NEG (Non-Evaporable Getters)
Several MEMS devices have been packaged by implementing active non-evaporable getters inside an anodically bonded cavity as described in the following articles:                H. Henmi, S. Shoji, Y. Shoji, K. Yosimi and M. Esashi, “Vacuum Package for Microresonators by Glass-Silicon Anodic Bonding”, INTERNATIONAL CONFERENCE ON SOLID STATE SENSORS AND ACTUATORS, TRANSDUCERS 93, pp. 584-587, 1993.        Y. Wang, M. Esashi, “The Structures for Electrostatic Servo Capacitive Vacuum Sensors”, SENSORS AND ACTUATORS, A66, pp. 213-217, 1998.        
U.S. Pat. Nos. 5,976,994 and 6,169,321 disclose a method and system for locally annealing a microstructure formed on a substrate.